Optical modulators that use a semiconductor, e.g., indium phosphide (InP), have been developed in recent years in the field of optical communication systems. Optical modulators that use a semiconductor can be miniaturized more easily than optical modulators that use a ferroelectric crystal, such as lithium niobate (LiNbO3). However, because an optical modulator that uses a semiconductor confines light strongly therein, an angle of divergence of modulated light emerging from an optical waveguide into space is relatively large. The modulated light emerging from the optical waveguide into space contains a signal beam and a monitoring beam for use in monitoring the signal beam. An increase in the angle of divergence of the modulated light can cause interference between signal beams or interference between monitoring beams and therefore is unpreferable.
Under the circumstances, to reduce the increase in the angle of divergence, collimating lenses may be arranged downstream of a substrate where optical waveguides are formed. The collimating lenses are typically arranged in one-to-one correspondence with optical waveguides that guide signal beams and optical waveguides that guide monitoring beams; each of the collimating lenses collimates the signal beam or the monitoring beam emerging from a corresponding one of the optical waveguides. The collimated signal beam and the collimated monitoring beam, each leaving a corresponding collimating lens, are directed in the same emergent direction.
However, when the signal beam and the monitoring beam, each leaving a corresponding collimating lens, are directed in the same emergent direction, a limitation can be imposed on a layout of one or more optical components downstream of the collimating lenses. For instance, an arrangement where a polarization beam combiner that polarization couples signal beams and light-receiving elements that receive monitoring beams are arranged downstream of the collimating lenses will impose a limitation on positional relationship between the polarization beam combiner and the light-receiving elements. Hence, it is preferable that a signal beam and a monitoring beam, each leaving a corresponding collimating lens, are directed in directions that differ from each other. Specifically, schemes currently under study include displacing an optical axis of each of collimating lenses that collimates a signal beam or a monitoring beam from an optical axis of a corresponding optical waveguide, thereby directing the signal beam and the monitoring beam leaving the collimating lenses in directions that differ from each other.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2015-169795
However, the configuration where the optical axis of each collimating lens that collimates the signal beam or the monitoring beam is displaced from the optical axis of the corresponding optical waveguide presumes that collimating lenses are arranged in one-to-one correspondence with optical waveguides. Hence, when the above-described configuration is applied to, for instance, an optical modulator, as the number of the optical waveguides increases, the number of the collimating lenses increases. This makes it difficult to miniaturize the optical modulator as desired. This holds true with apparatuses other than optical modulators as well. In an apparatus where collimating lenses are arranged downstream of a substrate where optical waveguides are formed, the collimating lenses are arranged in one-to-one correspondence with the optical waveguides, which disadvantageously limits miniaturization of the apparatus.